1. Field of the Invention
The present invention relates to a film provided with a hardcoat, which comprises a coating provided, for example, on the surface of various plastic films and has been improved in physical and chemical properties, for example, hardness, such as scratch resistance, and contamination resistance, and thus has high durability, and a process for producing the same.
2. Background Art
Films provided with a hardcoat, when used, are applied to various articles, and are colorless transparent or colored transparent as a whole so that the appearance of the articles can be seen or, when the films are applied, for example, to display equipment, the contents of display can be seen. These films have, on the surface thereof, a highly durable cured film which has been formed from a curable resin.
In the production of a film provided with a hardcoat, a heat-curable resin or an ultraviolet-curable resin is coated to form a coating which is then heated in the case of the former or is exposed to ultraviolet light in the case of the latter to cause crosslinking of the coating, thereby imparting necessary durability. In recent years, from the viewpoint of processing speed at the time of the production of the films, a method is in many cases adopted wherein an ultraviolet-curable resin is coated to form a coating which is then cured by ultraviolet irradiation.
The ultraviolet light is inferior to electron beams in the ability to pass through materials. Therefore, the interposition of an ultraviolet absorbing material lowers the ultraviolet irradiation efficiency and makes it impossible to satisfactorily cure the coating. This often results in the formation of a cured coating having somewhat lowered durability.
In this connection, the following fact should be noted. In the formation of a hardcoat, after the formation of an ultraviolet-curable coating on a transparent substrate film, ultraviolet light is applied to the coating to cure the coating. In this case, the application of the ultraviolet light through the transparent substrate film side is often inferior in irradiation efficiency to the application of the ultraviolet light from the coating side, because most of transparent substrate films contain an ultraviolet absorber for preventing a deterioration upon exposure to ultraviolet light.
For example, triacetylcellulose films, which are highly transparent to visible light and thus have been extensively used in optical applications, contain benzotriazole or other ultraviolet absorbers which absorb ultraviolet light with wavelengths around 340 to 350 nm.
The application of ultraviolet light from the ultraviolet-curable coating side can solve the problem of the absorption of ultraviolet light by the transparent substrate film. The application of ultraviolet light without covering the coating side, however, inhibits the polymerization of the coating due to the presence of oxygen in the air, and, thus, the degree of polymerization cannot be satisfactorily improved. For this reason, in some cases, the formed hardcoat does not have satisfactory durability. An attempt to cover the coating side, for example, with a transparent plastic film is likely to cause the coating to adhere to the transparent plastic film. Therefore, the transparent plastic film is not reusable and thus is wasteful.
In some cases, the film with a hardcoat is used in antireflection applications. In this case, a laminate of a plurality of layers, for example, metallic thin layers different from each other or one another in refractive index, are stacked on the hardcoat to form an antireflection film which, when used, is applied onto the surface of various displays.
In the case of this antireflection film for displays, the reflection of incident light from the front side can be prevented by the laminate of the plurality of metallic thin layers. On the other hand, regarding light from the backside, the incorporation of a light diffusing agent or the formation of fine concaves and convexes on the surface of the hardcoat is necessary from the viewpoint of preventing such an unfavorable phenomenon that the backside light derived from the display is seen in a highly bright state in specific sites.
In the former method, it is difficult, from the viewpoint of production control, to always incorporate the light diffusing agent in a given proportion.
On the other hand, in the latter method, concaves and convexes can be formed by putting a transparent concave/convex film on the ultraviolet-curable coating followed by the application of ultraviolet light through the concave/convex film. Also in this case, the coating material is likely to adhere to the concave/convex film, and, thus, the concave/convex film is not reusable and is wasteful.
When the coating is once cured followed by the formation of concaves and convexes using an emboss plate, the problem of the waste of the concave/convex film can be solved. In this case, however, since the coating is already in a cured state, the reproduction of concaves and convexes by embossing is unsatisfactory.
Further, although studies have been made on ultraviolet-curable resin compositions and curing methods for the formation of the hardcoat, a significant improvement could not have been achieved.
Accordingly, it is an object of the present invention to provide a film provided with a hardcoat which, at the time of the production of the film with a hardcoat, does not require the use of a transparent plastic film, which is likely to adhere to a coating, and has high durability, and to provide a film provided with a hardcoat which, despite the fact that the use of a transparent plastic film, which is likely to adhere to the coating, is not required at the time of the production of the film with a hardcoat, has a flat surface or has an embossed surface in which concaves and convexes of an embossing die have been faithfully reproduced.
It is another object of the present invention to improve the slipperiness of the film with a hardcoat without the use of any expensive material or without providing a complicate step and thus to improve apparent scratch resistance.
It is a further object of the present invention to provide a process for efficiently producing the above film provided with a hardcoat.
The object of the present invention could have been attained by incorporating, as a photopolymerization initiator, for example, a photopolymerization initiator, which is effective in the wavelength region of 350 to 450 nm, into an ultraviolet-curable layer stacked on a transparent substrate film in the production of a film provided with a hardcoat.
Further, the object of the present invention could have been attained by incorporating a slip agent into an ultraviolet-curable layer to be applied.
Furthermore, according to the present invention, the surface hardness of the hardcoat could have been further improved by adopting a method wherein, in curing the ultraviolet-curable layer, ultraviolet light is applied to the ultraviolet-curable layer from the transparent substrate film side followed by the application of ultraviolet light from the ultraviolet-curable layer side.
The ultraviolet irradiation in two stages in this way have permitted the slip agent to effectively bleed on the surface of the hardcoat, and the contemplated effect of the incorporation of the slip agent could have been satisfactorily attained.
According to a first aspect of the present invention, there is provided a film comprising: a transparent substrate film; and, stacked on the transparent substrate film in the following order, a hardcoat and a slip layer,
said hardcoat being formed of a cured product of a material comprising an ultraviolet-curable resin and a photopolymerization initiator which initiates the photopolymerization of the ultraviolet-curable resin upon exposure to ultraviolet light in its wavelength region other than the wavelength region absorbable by the transparent substrate film,
said slip layer containing a slip agent.
According to a second aspect of the present invention, there is provided a process for producing a film comprising a transparent substrate film and a hardcoat provided on the transparent substrate film, said process comprising the steps of:
forming, on the transparent substrate film, an ultraviolet-curable layer comprising an ultraviolet-curable resin and a photopolymerization initiator which initiates the photopolymerization of the ultraviolet-curable resin upon exposure to ultraviolet light in its wavelength region other than the wavelength region absorbable by the transparent substrate film;
covering the top surface of the ultraviolet-curable layer with an oxygen impermeable material; and
applying ultraviolet light in its wavelength region, which permits the photopolymerization initiator to initiate the photopolymerization of the ultraviolet-curable layer, through the transparent substrate film to cure the ultraviolet-curable layer.